usb: musb: fix bit mask for CSR in musb_h_tx_flush_fifo()

The datasheet says that MUSB_TXCSR_FLUSHFIFO is only valid when
MUSB_TXCSR_TXPKTRDY is set as well.

With this patch applied, the warning in this function does no longer
kick in when an USB soundcard is unplugged while the stream is active.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>